Mixing heads having a choke design have been used, in particular, for the preparation of reaction mixtures for the production of moldings in processes which require exact mixing of the reaction components from the beginning to the end of the continuous mixing procedure. Technological advances in the production of moldings require, to an increasing extent, however, that reaction components of a higher viscosity are processed at lower throughput capacities-to way below 50 g of reaction mixture/sec.
With these requirements, previously-known mixing heads (such as the mixing head disclosed in German Auslegeschrift No. 2,612,812 (corresponding to U.S. Pat. No. 4,141,470), the choke member of which is designed as a lifting valve, or the mixing head in German Offenlegungsschrift No. 2,847,504, in which the choke member is in the form of a rotary valve) have the disadvantage that the volume of the mixing chamber which is formed between the choke member and the discharge piston is too large for faultless mixing.
The cross section of the mixing chamber cannot be reduced because the diameter of the discharge piston must not fall below a certain minimum. A thin, completely-reacted film of mixture builds up between the discharge piston and the guide bore therefor, which film may block the stroke movement of the piston. If the piston is too small, it may bend due to the charging force which is applied and the mixing head may be destroyed.
In order to reliably rule out the possibility of a collision between the discharge piston and the choke member, the cleaning stroke of the piston may only begin once the choke member has come to rest in the cleaning position. For this reason, after the injection nozzles have been closed, the remainder of the mixture which is still in the mixing chamber cannot flow away via the passage in the choke and does not undergo a subsequent mixing in the outlet passage. The end product frequently exhibits a defective area, the volume of which corresponds to the volume of that remaining mixture.
Since the discharge pistons in the mixing heads of the prior art have necessarily had minimum diameters, as described above, choke members were therefore also required to have such widths or diameters that sufficiently thick structural pieces would be present on all sides of its passage. Due to these necessary dimensions, and the positioning of the bodies of the injection members on the sides of the mixing chamber in the mixing head housing, it has been impossible for the known choke members to be positioned close enough to the nozzle openings to reduce the volume of the mixing chamber.
The object of the present invention is to provide a mixing chamber having a volume which is as small as possible, in order to achieve perfect mixing even when higher-viscosity components are processed, including when there are lower throughput capacities.
This object is achieved in that the guide recess and the choke member therein have sections of reduced cross-sectional area toward the discharge piston guide bore and mixing chamber, and extend up between the injection members, ending just below and leaving the nozzle openings clear.